Download NEWTON`S FIRST LAW OF MOTION The law of INERTIA

NEWTON’S FIRST
LAW OF MOTION
The law of INERTIA
INERTIA - describes how much an
object tries to resist a change in
motion (resist a force)
inertia increases with mass
INERTIA EXAMPLES
Ex- takes more force to stop a bowling
ball than a basket ball going the same
speed (bowling ball tries more to keep
going at a constant speed- resists
stopping)
Ex- it takes more force to lift a full
backpack than an empty one (full
backpack tries to stay still more- resists
moving ; it has more inertia)
What the law of inertia says….
AKA Newton’s First Law of Motion
An object at rest, stays at rest
Because the net force is zero!
Forces are balanced (equal & opposite)
…an object in motion, stays in motion
Motion means going at a constant speed
& in a straight line
The net force is zero on this object too!
NO ACCELERATION!
…unless acted upon by a net force.
Only when this happens will you get
acceleration! (speed or direction changes)
Forces are unbalanced nownot all equal and opposite
there is a net force> 0N!
You have overcome the object’s inertia!
How does a traveling object move once all the
forces on it are balanced?
at a constant speed in a straight line
Why don’t things just move at a constant speed in
a straight line forever then on Earth?
FRICTION! Opposes the motion & slows things
down or GRAVITY if motion is in up/down
direction
These make the forces unbalanced on the object &
cause acceleration
We can’t get away from those forces on Earth!
(we can encounter some situations where they are
negligible though!)
HOW can you make a Bowling Ball
TRAVEL IN A CIRCLE?
When objects are moving at a constant
velocity, how many different directions do
they go?
ONE
What path shape do the moving objects
follow?
STRAIGHT LINE
HOW does the Bowling Ball
TRAVEL IN A CIRCLE?
You have to keep pushing the ball to the
center of the circle with a
CENTRIPETAL FORCE
DIRECTION THE
BALL WANTS TO GO
(A STRAIGHT LINE)
DIRECTION OF THE
CENTRIPETAL FORCE
Broom pushes ball in
towards the center,
making it go in a circle
HOW DO OBJECTS
TRAVEL IN A CIRCLE?
You have to continually apply a CENTRIPETAL
FORCE
Centripetal force = center seeking force
force with a direction towards the center of the
circle
The ball was trying to travel in a straight line at a
constant speed(aka constant velocity) but the
push force from the broom kept pushing it
toward the circle’s center
Moving Inertia aka MOMENTUM
Which is tougher to stop?
-A 40 kg skateboarder or an
80 kg skateboarder going 5 m/s?
Why?
-A 40 kg skateboarder going 2 m/s or a
40 kg skateboarder going 5 m/s?
Why?
Momentum- moving inertia
Which is harder to stop?
(which has more moving inertia?)
-A 40 kg skateboarder going 5 m/s or an
80 kg skateboarder going 2 m/s?
MOMENTUM- measures a moving object’s
resistance to a change in motion, depends
on the object’s MASS and VELOCITY
p = mv
Momentum = mass(kg) x velocity(m/s)
NOW ANSWER IT
A 40 kg skateboarder going 5 m/s or an
80 kg skateboarder going 2 m/s?
Skater #1
p = mv = 40 kg x 5m/s forward
= 200 kgm/s forward
Skater #2
p = mv = 80 kg x 2m/s forward
= 160 kgm/s forward
Skater 1 has more momentum!
Skater 1 is tougher to stop!
INELASTIC COLLISIONSObjects stick together when they collide
and the total momentum in the system stays
the same (is conserved).
Total energy in the system does NOT stay
the same!
Objects are often deformed in the collision
Examples*a rear-end car accident
*shooting a bullet into a
block of wood
ELASTIC COLLISIONSObjects bounce off each other when they
collide and the total momentum AND total
energy in the system is conserved!
Objects are never deformed
in the collision
Examples*Playing pool
*Newton’s Cradle
GET YOUR LETTERS READY!
1. BROOMBALL- Which direction do you
push on a bowling ball to get it to go
smoothly around a trashcan?
A. Forward
B. Backward
C. Toward the trash can
D. Away from the trash can
2. You are in a car going at a constant
speed and throw an apple straight up in
the air. The apple lands…
A. In front of your hand & hits your knees
B. In back of your hand & hits your chest
C. In your hand
D. To the left of your hand
3. You fly a plane & release the cargo at the
point shown. Which target will the cargo
hit?
target A
target B
target C
4. A book is on your dashboard. You turn
the car left around a curve and the
book…
A. Stays in the same spot on the dash
B. Goes left of its spot on the dash
C. Goes right of its spot on the dash
D. Flies upward
5. You kick a ball in outer space.
During the kick, the ball…
A. Accelerates upward
B. Goes at a constant velocity upward
C. Has balanced forces on it
D. None of the above
6. You kick a ball in outer space.
After the kick, the ball…
A. Accelerates forward
B. Goes at a constant velocity forward
C. Has a net force of 0N
D. Both B & C
7. A box is lowered by a rope at a constant
velocity. Which is true?
A. The net force on the box is 0 N
B. The rope’s tension force is equal
to the weight force of the box
C. The box will accelerate because
of Fg if you let go of the rope
D. All of the above
8. You are going forward at 30 mph in a sports
car. You hit the gas and quickly reach 50
mph. The cup sitting on the frictionless dash
goes…
A. 80 mph forward
B. 50 mph forward
C. 30 mph forward
D. 20 mph forward
E. 20 mph backward
9. Why does the moon orbit the Earth?
A. The moon is in a circular motion and
stays in a circular motion until acted
upon by an outside force
B. The moon is pushed in a circular motion
by the Fg from the sun as a centripetal
force
C. The moon would go in a straight line at a
constant velocity but the centripetal force
from the Earth’s gravity pulls on it to
change its direction
10. What is the direction of the centripetal
force on the moon?
A. Toward the Earth
B. Toward the moon
C. Toward the sun
D. Toward the front of the moon’s path
11. You are pushed with a force of 20N and
go from 0m/s to 4m/s in 2 seconds. How
fast are you going after your partner
stops pushing you?
A. 0m/s
B. 2m/s
C. 3m/s
D. 4m/s
E. 5m/s
12. Going 4m/s, you turn the leaf blower off,
briefly slide on the floor and then quickly turn
it back on. Now your motion …
A. Continues to get slower and slower
B. Speeds back up to a constant speed of 4m/s
C. Continues at a new slower constant speed
D. Accelerates and starts to curve
13. As you push on a hockey puck, it
accelerates from 5m/s to 12m/s.
If the ice is frictionless, when the puck
leaves the hockey stick it…
A.
B.
C.
D.
Goes back to its 5m/s
motion
Slows down to a stop
Continues at a constant
speed of 12 m/s
Continues to constantly
accelerate
14. What is the result of a
60 kg skater going 4 m/s south running
into an 80 kg skater going 3m/s north?
A. Both skaters go north after they hit
B. The skaters both stop
C. Both skaters go south after they hit
15. A meteorite is being accelerated
through space by a 10N force. Suddenly
it encounters a second force of 10N in
the opposite direction from the first
force.
The meteorite…
A. Is brought rapidly to a halt
B. Decelerates gradually to a halt
C. Continues at the speed it had when it
encountered the second force
D. Gets slower and then continues at a
constant speed.
16. You push against a metal door with all
your might. The door…
A. Pushes back with a greater force than you
B. Pushes back with a smaller force than you
C. Pushes back with the same force as you
D. Starts pushing with a greater force than
you, but then pushes at the same force as
you
17. A golfer tees off and hits his golf ball
with a club. Which of the following are true?
A. The club pushes the ball with more force
than the ball pushes on the club
B. The ball’s force diagram was unbalanced
when it was just sitting on the tee
C. The force of the push on the ball is equal to
the ball’s mass times its velocity
D. None of these are true
15. You are riding a galloping horse
and drop a ball. Where will the ball
land relative to the ground?
A. in front of the place it was dropped
B. directly below the place it was
dropped
C. behind the place it was dropped
D. it will never land
http://www.youtube.com/watch?v=KDVttzSqUSo
Galileo's Battle for the Heavens 7
DEMO
16. Why does the egg fall into the cup
instead of go flying like the tube?
A. An object at rest stays at rest because of
its inertia
B. An object in motion stays in the same
constant velocity motion because of its
momentum
C. An object accelerates because a net
force >0N acts on it